1. Field of the Invention
This invention relates to the field of graphic digitizer systems and, in particular, to a graphic digitizer system which utilizes an improved pointer position detection system.
2. Description of Related Art
Graphic digitizer systems are used in the art for translating the position of a point on a plan or drawing into coordinates recognizable by a computer. A number of well known graphic digitizer systems make use of a movable coil and a work surface defined by one or more receiving grids of conductors. The coil may be disposed within the tip of a pen-shaped instrument and an oscillator is provided which supplies an ac driving signal of predetermined frequency and magnitude to the coil, which is inductively coupled to the conductors of the grid. In accordance with well known principles of electromagnetic theory, ac signals are induced in the grid conductors at a magnitude and phase that depend on the location of the coil relative to the conductors. Generally, the signals induced in the conductors will have a magnitude that varies from zero at the coil center to a maximum at the periphery and tapering off beyond. Furthermore, the phase of the signals induced in the conductors at one side of the coil will be the opposite of (180 degrees displaced from) that of signals induced in the conductors at the other side.
While in previous systems the movable coil was powered by a cable connecting the oscillator and the ac driving signal to the coil, or an ac oscillator was located in the pointer together with a battery for power, recent systems have utilized the grid itself as a transmitting antenna by applying the ac driving signal to the grid and having a tuned circuit in a pen-shaped pointer which receives the ac power signal and then acts as a transmitter to induce ac positional signals in the grid, whose conductors are accessed through the use of multiplexer circuitry to couple the induced signals to detection circuitry to locate the pointer. Such a so-called passive pointer digitizer system is disclosed in U.S. Pat. No. 4,875,553 issued Nov. 7, 1989 to Yamanami et al., entitled "Position Detecting Apparatus." This system requires, however, that the same X or Y conductor of the grid which acts as a transmitter also acts as a receiver in order that positional information be obtained from the radiated and reradiated signal, and thus requires that the ac driving signal be removed for a sufficient period of time from the X or Y conductor of the transmitting grids before the same such X or Y conductor of the now receiving grids are accessed to enable confusing clutter and transients to subside and to enable the ac driving signal to sufficiently decay so as not to be detected by the X or Y conductors. This results in a discontinuous communication path and a 50% or less duty cycle for receiving energy from the pointer. In addition, since the same wire that radiated the signal must now receive it, only a limited signal-to noise ratio can be obtained and the system has a high sensitivity to outside noise sources.
Thus, it is a primary object of the present invention to provide an improved pointer position detection system.
It is another object of the present invention to provide an improved pointer position detection system that is highly efficient.
It is a further object of the present invention to provide an improved pointer position detection system which uses a full-time driving signal and has a 100% receive time and offers a continuous communication path between the pointer and the receiving system.
It is still another object of the present invention to provide an improved pointer position detection system which has a greatly improved signal-to-noise ratio.
It is a further object of the present invention to provide an improved pointer position detection system which has a high immunity to outside noise sources.